1. Field of the Invention
The present invention relates to amplifiers, and in particular, to efficient power amplifying circuits and methods that compensate for nonlinear distortions produced by a power amplifier between an input signal and an output signal.
2. Description of the Related Art
Amplifiers are used in a variety of applications requiring small signal amplification. Low distortion in amplifier output is of particular importance in applications requiring linear processing or reproduction of signals containing information in the amplitude and phase of a signal. Amplifier output should exhibit low distortion in amplitude and phase to maintain integrity of this information upon amplification. In other words, to maintain high signal fidelity, an amplifier should exhibit a near linear input-to-output characteristic over its operating range.
Linear amplification is of particular importance in communication devices that transmit amplified signals having information encoded in the amplitude and phase of the signal. Signals transmitted from a source to a destination are often modulated and amplified before they are transmitted. Several existing and prospective wireless digital communication systems are based on modulation schemes with both varying amplitude and phase are often referred to as linear modulation schemes. Compared to modulation schemes having only phase or frequency modulation, linear modulation schemes provide higher spectral efficiency for a given throughput (number of bits per Hz per second).
Unfortunately, in a communication device the spectral properties of a signal can only be preserved if the entire transmitter chain is linear. If the transmitter is not linear, intermodulation distortion (IMD) products will be generated in the transmitted signal causing spectral growth of the signal that interferes with users in adjacent channels. If the nonlinearity is particularly strong, signal integrity will be jeopardized and lead to increased bit-error rates in the receiver. However, in practice the output of an amplifier is nonlinear because the output eventually saturates at some value as the amplitude of the input signal is increased. As the amplifier output is driven into saturation, IMD products in the output increase resulting in undesirable distortion.
A traditional method of obtaining linear amplification is to use class A power amplifiers operating far below saturation. However, this type of operation is inefficient since a class A amplifier will dissipate power even with a zero input signal or in a quiescent condition. This is a particularly significant power drain in portable devices operating on battery power.
The power amplifier in a transmitter is the main contributor of distortion because the design is a tradeoff between linearity on one side and power efficiency on the other. Recent attention has been directed to design of power amplifier configurations having linearization circuitry applied to power efficient, but nonlinear power amplifiers to obtain both linear amplification and high power efficiency. One method of achieving increased linearity is by using envelope feedback circuits, such as those disclosed by H. Kosugi et al. in xe2x80x9cA High-Efficiency Linear Power Amplifier Using an Envelope Feedback Method,xe2x80x9d Electronics and Communications in Japan, Part 2, vol. 77, no. 3, 1994, pp. 50-57, and B. Shi et al. in xe2x80x9cLinearization of RF Power Amplifiers Using Power Feedback,xe2x80x9d Proceedings of the 49th IEEE Vehicular Technology Conference, May 1999, pp. 1520-1524, both of which are hereby incorporated by reference.
FIG. 1 shows a block diagram of an RF power amplifier configuration 100 illustrative of the principle of envelope feedback. As shown in FIG. 1, an RF input signal si is weighted in a variable gain amplifier (VGA) 124 prior to a nonlinear power amplifier (PA) 128 so as to achieve an overall linear behavior. The weighting is obtained by a feedback circuit provided between an input node 120 on which is impressed an input signal si and an output node 130 receiving an amplified reproduction so of the input signal si. Between node 122 and node 130, the input signal si is adjusted by amplification or attenuation by the VGA 124. The adjusted signal is output to node 126 and then supplied to PA 128 to produce amplified output so.
Amplifier configuration 100 includes an envelope feedback circuit to supply a signal for controlling the VGA 124. The feedback circuit includes a first signal path along node 132 and a second signal path along node 133. The output signal so is coupled by a coupler 131 from output node 130 to node 132. The decoupled output of so is then supplied to a fixed attenuator 134. The fixed attenuator 134 scales the RF output so and defines the linear gain of the topology (assuming a very large loopgain). The scaled output is then provided to regular or power squared envelope detector 136.
The input signal si on node 120 is similarly coupled by a coupler 121 to node 133, which in turn is coupled to a regular envelope or squared power envelope detector (D) 135. The detected input and scaled output (regular or power) envelope signals are subtracted in a differencing element 140, such as a difference amplifier, to produce an error signal re. The error signal is then filtered in loop filter 142, such as a low pass filter (LPF), and amplified in an amplifier 144. Using a summing element 146, an optional preset offset Kc also may be applied to the amplified error signal prior to the control of the VGA 124. The amplified error signal is then provided to control the amplification level of the VGA 124, which conditions the input signal si prior to input into the PA 128.
FIG. 2 shows a block diagram of a power amplifier configuration 200 that is illustrative of an alternative to using a VGA to control an output amplitude of a nonlinear power amplifier. Elements with like reference numerals and their corresponding functions are described above. As shown in FIG. 2, the output power of a PA 228 is directly controlled by the error signal re. In this case, the error signal re, for example, may control the operating point or the supply power voltage. Direct control of the PA 228, for example, by controlling the operating point, power supply and/or another direct control method, is a way of implementing VGA functionality within the PA.
As discussed below in detail, the linearity in the combined gain of the PA and the VGA may be adversely affected depending on the choice of loop component parameters. Thus, the effect that the loopgain has on the overall gain of the amplifier configurations of FIGS. 1 and 2 merits further investigation.
The techniques described above have the same feedback topology. What does differ between the implementations of FIG. 1 and FIG. 2 is the relationship between the VGA/PA control signal and the output amplitude of the PA. In any case, it can be assumed that this relationship will be more or less nonlinear. Thus, without any loss of generality, it is sufficient to consider the topology in FIG. 1 in the following analysis outlining effects that small and large signals have on the linearity of the PA in either feedback configuration.
With respect to the detectors 135 and 136, either regular envelope detectors or power detectors have been proposed. Starting with the envelope detector and considering the DC characteristics of the loop (i.e., disregarding the loop filter), the complex baseband equivalent of an ideal envelope detector is the absolute value (a real value) of a complex valued signal. Thus,
D(sx)=|sx|xe2x89xa1rxxe2x80x83xe2x80x83(equation 1), 
and for this case the output amplitude is given by                               r          o                =                                                            A                G                            ⁡                              (                                                      K                    c                                    +                                                            A                      c                                        ⁢                                          r                      i                                                                      )                                                    1              +                                                A                  G                                ⁢                                  A                  c                                ⁢                                  r                  i                                ⁢                β                                              ·                                    r              i                        .                                              (                  equation          ⁢                      xe2x80x83                    ⁢          2                )            
With AGAcxcex2ri greater than  greater than 1, where AGAcxcex2ri may be defined as the loopgain of the system, and for AGAcri greater than  greater than Kc, the output amplitude is given by                                           r            o                    ≈                                    r              i                        β                          ,                            (                  equation          ⁢                      xe2x80x83                    ⁢          3                )            
where 1/xcex2 may be defined as the linear gain of the system.
Note that the loopgain is proportional to the input signal amplitude. Thus, the condition AGAcrixcex2 greater than  greater than 1 will only be valid for a limited input amplitude range. After inspection of equation 2, the offset may be chosen as                                           K            c                    =                      1                          β              ⁢                              xe2x80x83                            ⁢                              A                G                                                    ,                            (                  equation          ⁢                      xe2x80x83                    ⁢          4                )            
which also results in                               r          o                ≈                                            r              i                        β                    .                                    (                  equation          ⁢                      xe2x80x83                    ⁢          5                )            
The forgoing analysis has not considered the fact that AG is a nonlinear function that is dependent on the input signal amplitude. However, if the PA 128 is linear for small input signals, Kc may be set to                                           K            c0                    =                      1                          β              ⁢                              xe2x80x83                            ⁢                              A                G0                                                    ,                            (                  equation          ⁢                      xe2x80x83                    ⁢          6                )            
where AG0 is the small-signal gain of the PA 128. Thereby, linear operation with desired gain will be obtained over the range for which the PA 128 is linear. Beyond this range, the loopgain must be sufficiently large (according to the discussion above) to give a linear PA 128 over the full range of input amplitudes.
This reasoning also applies when using power detectors. With
D(sx)=|sx|2xe2x89xa1rx2xe2x80x83xe2x80x83(equation 7), 
the output amplitude becomes                                           r            o                    =                                    1              β                        ·                          (                                                -                                      1                                          2                      ⁢                                              xe2x80x83                                            ⁢                                              A                        G                                            ⁢                                              A                        c                                            ⁢                      β                      ⁢                                              xe2x80x83                                            ⁢                                              r                        i                                                                                            +                                                                                                    (                                                  1                                                      2                            ⁢                                                          A                              G                                                        ⁢                                                          A                              c                                                        ⁢                            β                            ⁢                                                          xe2x80x83                                                        ⁢                                                          r                              i                                                                                                      )                                            2                                        +                                                                  K                        c                                                                    A                        c                                                              +                                          r                      i                      2                                                                                  )                                      ,                            (                  equation          ⁢                      xe2x80x83                    ⁢          8                )            
and equation 6 still applies to obtain the desired gain for small input amplitudes, whereas the loopgain AGAcxcex2ri must be large beyond the small-signal range. The loopgain referred to above is the large signal loopgain, which is useful when considering the linearity of the system. From a stability point of view (as it is a feedback system) the differential loopgain should be considered.
A convenient way of extending the linear range of the topologies discussed above is setting the optional offset Kc to a proper value according to equation 6. However, as discussed below in more detail, if Kc does not have a correct value, the topologies will become nonlinear for small input signals even if the PA 128 is ideally linear.
Another way to extend the linear range of the foregoing topologies would be to increase the loopgain AGAcxcex2ri instead of setting offset Kc to a desired value, as the latter may not be well defined. However, increasing AGAcxcex2ri would come at the expense of a reduced maximum bandwidth because the product of the loopgain, the loop bandwidth and the loop delay determines the phase margin of the feedback system. Thus, a reduction in loopgain can be traded for an increased bandwidth. Consequently, for a given bandwidth there exists a maximum loopgain which may result in insufficient loopgain for low input amplitudes. Therefore, to optimize the bandwidth, Kc must be set to its proper value.
Unfortunately, there is a problem in setting a value for Kc, once and for all, because the small signal gain of the power amplifier is not sufficiently accurate. Hence, there is a need for an adaptive approach to setting the value of Kc to account for such variations.
The effects of various amplifier parameters values are now described. FIG. 3a shows the effect that the feedback loop has on gain (ro/ri) for a PA linearization configuration having a regular envelope detector and using the following parameters: AG=10; xcex2=0.1; 0xe2x89xa6r, xe2x89xa61; and Kc=0. Similarly, FIG. 3b shows the gain for a PA linear configuration with the same parameter values, but using power detectors instead of regular envelope detectors. In both of FIGS. 3a and 3b, a linear PA is assumed to simplify evaluation of the region where the loopgain becomes low and where the PA should otherwise be linear. The parameter Kc was set to zero while the gain Ac of amplifier 144, which basically constitutes the peak loopgain of these configurations, was set to various values to demonstrate the poor behavior of the loop for low input amplitudes.
As shown in FIGS. 3a and 3b, when no offset Kc is provided in the feedback loop, very large values of Ac may be required to obtain a constant gain for a reasonable input signal amplitude range. However, in most cases, such high values of Ac will not be required to compensate for the PA nonlinearities in the saturation region. In addition, Ac should be kept as low as possible, as discussed above, to maximize the bandwidth (without violating stability conditions) of the loop, and thus allow for larger signal bandwidths.
Next, the effects of various values of offset Kc with the gain Ac fixed at 10 are shown in FIGS. 4a and 4b for the linearization configuration of FIG. 1 using regular envelope detectors and power envelope detectors, respectively. As can be seen from FIGS. 4a and 4b, a high degree of accuracy is required for setting offset Kc. Note that the gain for zero input signal amplitude always equals Kcxc2x7AG0.
In FIGS. 5a and 5b, a simple nonlinearity is introduced to model a typical AMxe2x80x94AM saturation characteristic of a PA. The saturation characteristic is given by the following third-order real-valued polynomial:                                           r                          o              ,              pa                                =                      10            ·                          (                              1                -                                                      4                    27                                    ·                                      r                                          i                      ,                      pa                                        2                                                              )                        ·                          r                              i                ,                pa                                                    ,                            (                  equation          ⁢                      xe2x80x83                    ⁢          9                )            
where ro,pa is the amplitude of the signal output from the power amplifier and ri,pa is the amplitude of the signal input to the power amplifier. In the characteristic defined by equation 9, the output ro,pa saturates for ro,pa=10 and provides a small-signal gain of AG0=10. As shown in FIGS. 5a and 5b, the gain for the loop with envelope and power detectors, respectively, are shown for various setting of Kc and Ac. The special case with Kc=Kc0, Ac=0 corresponds to the gain of the PA alone.
The plots of FIGS. 3a to 5b show that offset Kc is just as important a parameter as the loopgain in the feedback system. As shown for small signal amplitudes in FIGS. 3a and 3b, a high loopgain can never make up for an incorrectly set offset Kc because the loopgain is proportional to the input signal and steadily drops with decreasing input signal amplitude. Thus, the offset Kc must be set accurately in an envelope feedback system to avoid distortion of the small signal portion of the input range.
Accordingly, the present invention is directed to automatic optimization of linearity of envelope feedback RF amplifier linearization that substantially obviates one or more of the problems due to the limitations and disadvantages of the related art.
In one aspect of the present invention, an amplifier configuration includes a power amplifier having a feedback loop for linearizing the power amplifier output. The feedback loop includes an offset value that is combined with an error signal that is a function of the input and output signal amplitudes and generated in the loop. If the input signal amplitude of the amplifier configuration is at or below a predetermined small signal value, the value of the offset parameter is adaptive to changes between the amplitudes of the input and output signals to linearize the power amplifier within this range. When amplitudes of the input signal are above the predetermined small signal threshold level, the offset value is maintained at a constant predetermined offset signal value.
Additional aspects and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned from practice of the invention. The aspects and advantages of the invention will be realized and attained by the system and method particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and exemplary only and are not restrictive of the invention, as claimed.
It should be emphasized that the terms xe2x80x9ccomprisesxe2x80x9d and xe2x80x9ccomprising,xe2x80x9d when used in this specification, are taken to specify the presence of stated features, integers, steps or components; but the use of these terms does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof